Global temperatures from space and the surface: why the discrepancy?

by Bob Henson
UCAR Communications

According to NASA's Goddard Institute for Space Studies (GISS), the globally averaged temperature from surface observations for 1996 was the fifth warmest this century. The global average calculated by GISS has been climbing roughly one-tenth of a degree Centigrade per decade since the 1970s. However, the readings obtained in the lower troposphere by NASA's satellite-based Microwave Sounder Units (MSUs) tell a different tale. They rank 1996 as only the 12th warmest in the 18 years since MSU data have been collected. In fact, the 1979-96 MSU trend is slightly negative (-0.03 degrees C per decade).

Global annual mean temperature anomalies as measured by surface data (black bars) and the MSU satellite unit (gray bars) using the MSU-2R technique. Temperatures on the vertical axis are times 100 degrees C. (Illustration from Hurrell and Trenberth, "Satellite versus surface estimates of air temperature since 1979," Journal of Climate 9(9), 1996.)

The layperson might ask, "So what?" A difference on the order of 0.13 degrees C per decade may not sound like much, but it is vitally important in assessing global temperature change. A global warming of one or two degrees over the next century--considered likely by the Intergovernmental Panel on Climate Change--could produce major climatic upheavals. If the surface and MSU trends continued over a century, the two records would disagree by almost as much as the IPCC projection itself.

A flurry of papers over the past several years in the Journal of Climate, Climatic Change, and other literature has tried to pin down the causes of the MSU/surface disagreement. At last month's meeting of the American Meteorological Society (AMS) in Long Beach, California, an afternoon session was devoted to the discrepancies. The lively session provided a thorough airing of the issues at hand, but no clear resolution.

Two key players in the debate are NCAR's James Hurrell and Kevin Trenberth. They have collaborated on a series of recent papers (including two presented at AMS) aimed at reconciling the patterns of difference between the surface and the MSU data. "We need both surface and MSU records to get a proper perspective on what's going on," says Trenberth. "Although the MSU data are excellent for many purposes, we think there are some substantive problems with the trends as depicted by MSU."

John Christy (University of Alabama at Huntsville) and Roy Spencer (NASA) have led the development of temperature-trend retrieval from MSU data. Both presented papers at AMS on their most recent work. Christy maintains that the MSU data are solid. "Since the MSU measures temperatures through a deep layer of the atmosphere, the only proper comparison is with temperature profiles from radiosondes. Our extensive comparisons show no significant disagreements." Christy believes the main difference between surface and MSU readings lies in the very real distinction between surface and tropospheric temperature, an area now receiving increased scrutiny.

Readings above and below

MSUs began collecting data from a NOAA polar-orbiting Television Infrared Observing Satellite (TIROS) in December 1978. Since then, MSUs have been launched aboard eight subsequent NOAA satellites. Each unit measures the brightness of oxygen emissions, from which the temperature of the oxygen (and thus the atmosphere) through a broad vertical layer can be inferred. The two MSUs now in space collect 30,000 observations daily from four channels between the surface and 30 km. To gain a single temperature, the MSU data from different viewing angles of one channel (channel 2) are combined in a way to enhance the signal from the lower troposphere. Dubbed MSU-2R (retrieval of channel 2), this technique uses emissions measured in the lowest 7 km of the atmosphere.

In the MSUs' favor is their strong correlation with global radiosonde records. Christy notes the minimal difference in decadal temperature trends between MSU-2R and radiosonde data across North America, the Arctic, and the tropics. In all three regions, Christy has found a difference of 0.027degreesC or less per decade between the radiosonde and MSU trends.

Although it has longevity on its side, the earth's surface temperature record has well-documented flaws. Tom Karl (National Climatic Data Center) found in 1994 that about 10% of the half-degree C warming seen at the surface this century could be due to error induced by the uneven distribution of weather stations across land and by their absence over the oceans, where sea-surface temperatures (SSTs) are routinely used as a substitute for surface air temperature. Karl believes the error could be proportionately greater over shorter intervals of a decade or two.

Adding to the murkiness, a number of recent earth-system upheavals have had global impact; in particular, the major El Niño/Southern Oscillation (ENSO) event of 1982-83 and volcanoes El Chich—n (1982) and Mt. Pinatubo (1991). Philip Jones (University of East Anglia) estimates that the effect of these transitory events could be twice as large in the troposphere as at the surface, with El Niño warming and volcanic cooling both amplified aloft.

Keeping tabs on the tropics

The tropics, where ENSO and recent volcanic effects are most pronounced, rest at the center of the MSU/surface debate. Temperatures at midlatitudes are highly variable, so subtle trends can be masked by natural variability. In the tropics, however, large-scale temperature changes are modulated by sea-surface temperature, and thus long-term trends are easier to discern.

John Christy

In examining MSU temperatures above the tropics from 1979 to 1995 using statistics and climate models, Hurrell and Trenberth found that most of the MSU temperature variation could be explained by SSTs. After they removed the SST effects, two significant temperature drops remained, one in 1981 and the other in 1991. Both drops, they believe, may be connected to transitions between NOAA satellites that occurred in those years.

Correction factors must be applied to each MSU to account for different satellite orbits and orbital shifts over time. Moreover, as much as 20% of the MSU-2R readings above land areas, and up to 10% above oceans, come from surface-based emissions. Hurrell and Trenberth argue that unusual land characteristics at the time of satellite transition, such as those caused by persistent droughts or floods, may have altered the surface emissions and biased the MSU-2R trends.

Christy's MSU-to-radiosonde comparisons show little disagreement in the tropics, although Hurrell and Trenberth question the reliability of radiosonde records there, where site changes and other confounding events are frequent. Instead of satellite transitions, Christy suspects natural features are responsible for the 1981 and 1991 temperature drops. He cites a 1981 African volcano, Nyamuragira, along with 1982's El Chich—n and 1991's Mt. Pinatubo. However, MSU temperatures have been slow to return to their pre-Pinatubo values, while surface temperatures have rebounded completely. Indeed, even with the influence of ENSO and volcanoes factored out, the MSU and surface readings continue to reflect increasing disparity over time.

Improved computer models may help clarify things. Hurrell and Trenberth are now using NCAR's latest community climate model, CCM3 (part of the NCAR climate system model), and new global reanalyses produced by the National Center for Environmental Prediction and NCAR's Scientific Computing Division. Christy has reservations about using climate models to project global temperature trends: "These vertical variations being modeled are very small, and they are affected by such factors as volcanic aerosols, which are not included explicitly in CCM3 and similar models." Trenberth argues that volcanic effects are reflected in the sea-surface data used as input to study the 1979-95 period. "CCM3 is indeed suitable for this research, and it in fact accounts for the vast majority of the temperature variability we've found."

Physics at work?

Should the tropospheric and surface temperatures necessarily move in lockstep from year to year? The standard lapse rate, or the vertical drop in temperature from the bottom to the top of the troposphere, is more than 75degreesC. In theory, thunderstorms and other convective air motions ought to ensure that the air is well mixed on a global scale, maintaining this lapse rate over time. Yet all parties acknowledge that changes can and do happen in the atmosphere's vertical temperature structure (for instance, ozone depletion has produced record cold in the lower stratosphere during the 1990s).

James Hurrell

"The problem occurs when you assume that the surface and tropospheric temperatures will behave identically," says Christy. Hurrell agrees: "We're not saying that the trends at all different levels of the atmosphere should be identical. Changes in lapse rates can occur over time." However, he adds, "it seems unlikely that there have been changes in area-averaged lapse rates at the same points in time where we identified the MSU temperature shifts."

Some of the work by Hurrell and Trenberth has focused on differences in the vertical profiles between land and ocean. The strongest surface warming of the past few years has been found over Northern Hemisphere continents, particularly in winter, while MSU's slight cooling trend has been centered above the oceans. Hurrell points out that wintertime cold fronts sweeping from land to sea are modulated near the relatively warm ocean yet maintain their strength higher up. In contrast, over land, inversions often trap cold air near the surface while the troposphere above remains warmer. The prevalence of these inversions means that a reduction in their number or strength (through increased low-level water vapor, for example) could produce a significant warming of average surface temperature with little change at higher levels. Moreover, the inversions are often too shallow to be diagnosed or tracked by computer models.

Kevin Trenberth

NASA is preparing to launch the first of its advanced MSUs (AMSUs) later this year. Instead of the four observing channels of the current model, the AMSU will have 16. "They'll be at slightly different frequencies," says Christy, "so they'll be measuring slightly different depths. Our job will be to reconcile the old and new records so they are compatible." Meanwhile, Christy has produced a newer version of the original MSU-2R data set to address some known inconsistencies. The revision (too recent to be used in the Hurrell/Trenberth work) shows slightly more tropospheric warming than before, although Hurrell says "the discrepancies we discovered still exist."

Those involved in the global temperature game, and others on the sidelines, are paying keen attention as the latest numbers roll in. Says Spencer: "The surface and satellite records cannot continue to diverge indefinitely. They should gradually come closer to one another." Both records are now seen as critical to unraveling global change, especially with an atmosphere acting contrary to some modeling results and theoretical assumptions. As Christy puts it: "Do we have an atmosphere that's less rigid than we think?"